1. Field of the Invention
The present invention relates to an image reading apparatus configured to read a document and a control method thereof.
2. Related Background Art
While data has been conventionally stored in paper, data can be readily computerized and stored at low cost in recent years, since personal computers (PCs) are commonly used and highly functionalized.
Some scanners (image reading apparatuses) are equipped with an automatic document feeder (ADF) to readily obtain an electronic file at high speed without manually feeding individual documents. The reading speed of the ADF is one of significant specifications for users, along with image quality.
When the document reading speed of the ADF exceeds a speed of transferring image data to a host computer, the read image data is temporarily stored in a storage unit (hereinafter referred to as a buffer memory) of the scanner to be subsequently transferred.
When the buffer memory has become full, image reading is temporarily suspended until the buffer memory becomes empty. When the buffer memory has become empty, image reading is resumed.
Interrupting read processing of a document as described above is, hereinafter, referred to as start/stop processing (SS processing). When SS processing is performed, document transfer for reading is suspended. Therefore, the occurrence of SS processing results in an increased amount of time required for reading a document.
However, along with a tendency toward higher resolution in recent years, the amount of data to be transferred may become enormous. Therefore, SS processing tends to occur more frequently. The frequency of occurrence of SS processing depends on a data transfer capacity to a host computer.
The data transfer capacity depends on various configuration elements, such as the type/speed of a central processing unit (CPU), a memory size, the capacity/speed of a hard disk drive (HDD), and the transfer speed for each communication method.
Further, the image reading apparatus may include a copying function, such as in a copying machine, in addition to the function of a personal computer (PC) scan for transferring image data read by an image scanner to a host computer.
Furthermore, even when a document is read using the ADF provided in a device (peripheral device or peripheral apparatus) having a facsimile (FAX) sending function, SS processing may also occur.
When SS processing occurs, an image continuity may be lost to deteriorate image quality. However, as long as the document is stably conveyed in a document conveyance path, the deterioration of image quality caused by SS processing can be reduced to some extent.
There can be some points in the document conveyance path that cause a document to move unstably. In this case, when SS processing is performed at such a point, image data read before stopping and image data read after resuming may not be smoothly joined, so that image quality may be greatly deteriorated.
In order to prevent image data from being deteriorated by the occurrence of SS processing, U.S. Pat. No. 6,648,226 discusses a method of changing the reading speed of an image reading apparatus according to a transfer time from an image reading apparatus to a host computer. When a long time is required to transfer image data to the host computer, the amount of stored image data in a buffer random access memory (RAM) is reduced by decreasing the reading speed, thus reducing the frequency of occurrence of SS processing.
Japanese Patent Application Laid-Open No. 11-239251 discusses a method of temporarily stopping reading when the free space of a buffer memory in a reading apparatus is a certain value or less. Further, Japanese Patent Application Laid-Open No. 11-239251 discusses a method of determining whether the image type is a white image or a black image during reading.
Japanese Patent Application Laid-Open No. 2000-287045 discusses that, if reading is interrupted, a switchback operation is performed depending on the position of a reading sensor unit when reading is resumed after interruption.
The occurrence of SS processing occurrence cannot be prevented according to the above-described methods, which can only reduce the frequency of occurrence of SS processing by decreasing the reading speed. SS processing may occur when data processing is seriously delayed in a host computer or when the transfer speed of a data transfer unit decreases for some reasons.
In this case, at some positions where a document stops in a document conveyance path of the reading apparatus, the document is neither smoothly conveyed nor smoothly read when reading is resumed, so that the quality of image data may be deteriorated.